Zoom lens and image projection apparatus including the same

ABSTRACT

A zoom lens includes, in order from an enlargement conjugate side to a reduction conjugate side, a first lens unit of negative refractive power; a second lens unit of positive refractive power; a third lens unit of positive refractive power, the third lens unit having at least three lens elements; a middle lens unit including at least one lens unit; and a last lens unit of positive refractive power. All the lens units, except the first lens unit and the last lens unit, move during zooming. The zoom lens satisfies appropriate conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/365,482 filed Feb. 4, 2009, which claims the benefit of JapanesePatent Application No. 2008-023848 filed Feb. 4, 2008, all of which arehereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens and an image projectionapparatus including the zoom lens. For example, the present inventionrelates to a zoom lens having a long back focus, providing a high levelof optical performance, and suitable for a projector producing aprojected image.

2. Description of the Related Art

There have been proposed a variety of projectors (image projectionapparatuses) using a display device, such as a liquid crystal displaydevice, to project an image formed on the display device onto a screensurface.

A projection lens included in such a projector is desired to have suchcharacteristics as follows.

In a typical color projector including three display devices, light froma white light source is separated by a color separation optical systeminto light beams of red, green, and blue colors, which are guided to thecorresponding display devices. The light beams ejected from the displaydevices are synthesized by a color synthesis optical system and areincident on a projection lens. In this configuration, to provide a prismor the like for synthesizing the color light beams passed through thedisplay devices, it is necessary to leave a space between the displaydevices and the projection lens. Therefore, it is desirable that theprojection lens have a certain length of back focus.

If the angles of light beams incident on the color synthesis opticalsystem from the display devices vary, the spectral transmittance of thecolor synthesis optical system varies accordingly. As a result, thebrightness of each color in a projected image varies depending on thefield angle and thus, the projected image is difficult to see. To reducesuch an effect depending on the field angle, it is desirable that apupil on the display device side (reduction conjugate side) besubstantially at infinity, that is, so-called telecentricity be good.

When pictures (images) on the display devices for three different colorsare synthesized and projected onto a screen, the perceived resolution ofdisplayed characters or the like is reduced if, for example, they lookdouble. To prevent this, it is necessary that pixels of the respectivecolors be superimposed on one another throughout the screen. It is thusdesirable that color misregistration (lateral chromatic aberration) thatoccurs in the projection lens be effectively corrected across thevisible light spectrum.

To prevent a projected image from being distorted and becoming difficultto see, it is desirable that distortion be effectively corrected.

As an example of a zoom lens for a projector, there is known a zoom lensincluding a first lens unit disposed closest to the enlargementconjugate side, having negative refractive power, and fixed duringzooming; and a last lens unit disposed closest to the reductionconjugate side, having positive refractive power, and fixed duringzooming.

As an example of the zoom lens of this type, there is known a zoom lensin which at least three zoom lens units movable during zooming aredisposed between the first lens unit and the last lens unit (see, e.g.,U.S. Pat. No. 6,545,817).

In the zoom lens disclosed in U.S. Pat. No. 6,545,817, the second andthird lens units disposed in this order from the enlargement conjugateside perform a primary zoom function. In this zoom lens, image planevariation during zooming is corrected by moving the third lens unit andother lens units that follow. In this type of zoom lens, the third lensunit is a very important lens unit for providing a high level of opticalperformance throughout the zoom range. Therefore, the third lens unithas at least three lens elements to improve optical performance.

A zoom lens for a projector is desired to have a long back focus and betelecentric on the reduction conjugate side. Therefore, a lens unit ofnegative refractive power is disposed on the enlargement conjugate sideand a lens unit of positive refractive power is disposed on thereduction conjugate side.

With the lens configuration described above, since the entire zoom lensbecomes asymmetric, it is difficult to provide a high level of opticalperformance throughout the zoom range. For example, negative distortionoften occurs at the wide-angle end, and it is very difficult to correctit.

In the zoom lens disclosed in U.S. Pat. No. 6,545,817, the third lensunit has a greater share of zoom ratio than those of the other lensunits. Accordingly, the third lens unit has refractive power greaterthan that of the other lens units and causes frequent occurrence ofaberration variation during zooming. Moreover, due to the greatrefractive power of the third lens unit, a large amount of negativedistortion at the wide-angle end tends to remain.

In the above-described zoom lens for a projector, it is important to setan appropriate lens configuration so as to ensure a predetermined zoomratio, effectively correct aberration variation during zooming, andprovide a high level of optical performance throughout the zoom range.

For example, it is important to appropriately define the refractivepower of each lens unit (the second and third lens units, in particular)and the lens configuration of the third lens unit. If the configurationof the second and third lens units is not appropriate, it is difficultto effectively correct distortion at the wide-angle end and provide ahigh level of optical performance throughout the zoom range.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided azoom lens including, in order from an enlargement conjugate side to areduction conjugate side, a first lens unit of negative refractivepower, a second lens unit of positive refractive power, a third lensunit of positive refractive power, a middle lens unit having at leastone lens unit, and a last lens unit of positive refractive power. In thezoom lens, all the lens units, except the first lens unit and the lastlens unit, move during zooming. The third lens unit has at least threelens elements. The following conditions are satisfied:

f3/fw>4.0

f3/f2>2.0

fw/Bf<0.8

where f2 is a focal length of the second lens unit, f3 is a focal lengthof the third lens unit, fw is a focal length of the entire zoom lens ata wide-angle end, and Bf is an air-equivalent back focus (anair-conversion length of back-focus).

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a lens configuration at a wide-angle end of a zoomlens according to a first exemplary embodiment of the present invention.

FIG. 2A and FIG. 2B are aberration diagrams of the zoom lens accordingto the first exemplary embodiment.

FIG. 3 illustrates a lens configuration at a wide-angle end of a zoomlens according to a second exemplary embodiment of the presentinvention.

FIG. 4A and FIG. 4B are aberration diagrams of the zoom lens accordingto the second exemplary embodiment.

FIG. 5 illustrates a lens configuration at a wide-angle end of a zoomlens according to a third exemplary embodiment of the present invention.

FIG. 6A and FIG. 6B are aberration diagrams of the zoom lens accordingto the third exemplary embodiment.

FIG. 7 schematically illustrates a main part of an image projectionapparatus according to an exemplary embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

The present invention provides a zoom lens capable of effectivelycorrecting various aberrations during zooming, offering excellentoptical performance throughout the zoom range, and suitable for aprojector. Zoom lenses according to exemplary embodiments of the presentinvention will now be described.

A zoom lens according to each exemplary embodiment of the presentinvention includes, in order from an enlargement conjugate side to areduction conjugate side, a first lens unit of negative refractivepower, a second lens unit of positive refractive power, a third lensunit of positive refractive power, a middle lens unit having at leastone lens unit, and a last lens unit of positive refractive power. Thefirst lens unit is closest to the enlargement conjugate side, while thelast lens unit is closest to the reduction conjugate side. In otherwords, the zoom lens of each exemplary embodiment has practically nolens unit other than the first lens unit, second lens unit, third lensunit, middle lens unit, and last lens unit. The enlargement conjugateside may be referred to as front (front side) or screen side. Thereduction conjugate side may be referred to as rear (rear side) or panelside (original image side, display device side, or liquid crystal panelside).

If the zoom lens includes five lens units (the first, second, third,fourth, and fifth lens units in order from the enlargement conjugateside), the middle lens unit corresponds to the fourth lens unit (havingpositive or negative refractive power). If the zoom lens includes sixlens units (the first, second, third, fourth, fifth, and sixth lensunits), the middle lens unit includes, in order from the enlargementconjugate side to the reduction conjugate side, the fourth lens unit ofnegative refractive power and the fifth lens unit of positive refractivepower. Alternatively, when the zoom lens includes six lens units, boththe fourth lens unit and the fifth lens unit may have positiverefractive power.

All the lens units, except the first lens unit and the last lens unit,move during zooming. All the moving lens units move toward theenlargement conjugate side during zooming from the wide-angle end to thetelephoto end. The second and third lens units perform a zoom function.In particular, the second lens unit performs a primary zoom function.The middle lens unit corrects image plane variation during zooming.

FIG. 1 schematically illustrates a main part of an image projectionapparatus (liquid crystal video projector) including a zoom lensaccording to a first exemplary embodiment of the present invention.

FIG. 2A and FIG. 2B are aberration diagrams showing aberrations at thewide-angle end (short focal length side) and the telephoto end (longfocal length side), respectively, of the zoom lens of the firstexemplary embodiment when a projection distance (from a first lens unitto a screen) is 2.1 m. In FIG. 2A and FIG. 2B, values in numericalexample 1 (described below) corresponding to the first exemplaryembodiment are expressed in millimeters (mm).

FIG. 3 schematically illustrates a main part of an image projectionapparatus including a zoom lens according to a second exemplaryembodiment of the present invention.

FIG. 4A and FIG. 4B are aberration diagrams showing aberrations at thewide-angle end and the telephoto end, respectively, of the zoom lens ofthe second exemplary embodiment when a projection distance is 2.1 m. InFIG. 4A and FIG. 4B, values in numerical example 2 (described below)corresponding to the second exemplary embodiment are expressed in mm.

FIG. 5 schematically illustrates a main part of an image projectionapparatus including a zoom lens according to a third exemplaryembodiment of the present invention.

FIG. 6A and FIG. 6B are aberration diagrams showing aberrations at thewide-angle end and the telephoto end, respectively, of the zoom lens ofthe third exemplary embodiment when a projection distance is 2.1 m. InFIG. 6A and FIG. 6B, values in numerical example 3 (described below)corresponding to the third exemplary embodiment are expressed in mm.

FIG. 7 schematically illustrates a main part of an image projectionapparatus according to an exemplary embodiment of the present invention.

In the image projection apparatus illustrated in FIGS. 1, 3, and 5, azoom lens (projection lens) PL enlarges and projects an original imageLCD (on a display device or liquid crystal panel) onto a screen surfaceS serving as a projection surface. In FIGS. 1, 3, and 5, an arrow undera lens unit indicates both the direction and amount of movement of thelens unit during zooming from the wide-angle end (wide end) to thetelephoto end (tele-end). The greater the inclination of the arrowrelative to the vertical direction, the greater the amount of movementof the lens unit from the wide-angle end to the telephoto end.

Reference character i denotes the order of the lens unit from theenlargement conjugate side. Reference character Li denotes the i-th lensunit.

Reference character L1 denotes a first lens unit of negative refractivepower, reference character L2 denotes a second lens unit of positiverefractive power, reference character L3 denotes a third lens unit ofpositive refractive power, reference character LM denotes a middle lensunit including at least one lens unit, and reference character LRdenotes a last lens unit of positive refractive power.

Reference character S denotes a screen surface (projection surface,enlargement conjugate surface, or enlargement-side conjugate position),reference character LCD denotes an original image (projected image) on aliquid crystal panel (liquid crystal display device, reduction conjugatesurface, or reduction-side conjugate position). The screen surface S andthe original image LCD have a conjugate relationship or substantiallyconjugate relationship therebetween. Generally, the screen surface Scorresponds to a conjugate point (enlargement conjugate point or frontside) having a longer distance, while the original image LCD correspondsto a conjugate point (reduction conjugate point or rear side) having ashorter distance. When the zoom lens is used as a photographing system,the screen surface S corresponds to an object side and the originalimage LCD corresponds to an image side.

Reference character GB denotes a glass block (prism) provided foroptical design purposes and corresponding to a color synthesis prism,polarizing filter, color filter, and the like.

The zoom lens PL is mounted on a projector main body (not shown) with aconnecting member (not shown) interposed therebetween. The glass blockGB and other components upstream of the zoom lens PL and adjacent to thedisplay device LCD are contained in the projector main body.

The zoom lenses of the first to third exemplary embodiments haveF-numbers in the 1.85 to 2.65 range. Each of the zoom lenses projectsimage information (display image) onto the screen surface S at aprojection distance of 2.1 m (when values in the corresponding numericalexample are expressed in mm).

In the aberration diagrams of FIGS. 2A, 2B, 4A, 4B, 6A, and 6B, a curved represents aberration at d-line (587.6 nm), a curve F representsaberration at F-line (486.1 nm), and a curve C represents aberration atC-line (656.3 nm).

Also, in FIGS. 2A, 2B, 4A, 4B, 6A, and 6B, a curve S (tilt of a sagittalimage plane) and a curve M (tilt of a meridional image plane) bothrepresent aberration at a wavelength of 550 nm, F indicates an F-number,and Y indicates an image height (image height on the projected side).

In each exemplary embodiment, the second lens unit L2 serves as aprimary zoom lens unit. The third lens unit L3 has refractive powersmaller than that of the second lens unit L2.

It is desirable that the third lens unit L3 include at least three lenselements. In the first, second, and third exemplary embodimentsdescribed below, the third lens unit L3 includes, in order from theenlargement conjugate side, a positive lens element G31, a negative lenselement G32, and a positive lens element G33 (two lens elements on theenlargement conjugate side constitute a cemented lens component).However, the lens configuration of the third lens unit L3 is not limitedto this. For example, the third lens unit L3 may include four or fivelens elements. More specifically, the positive lens element G33 closestto the reduction conjugate side may be replaced with a cemented lenscomponent formed by cementing together a positive lens element and anegative lens element. Alternatively, another positive or negative lenselement may be added to the reduction conjugate side (or enlargementconjugate side) of the positive lens element G33.

The following conditions are satisfied:

f3/fw>4.0  (1)

f3/f2>2.0  (2)

fw/Bf<0.8  (3)

where f2 is a focal length of the second lens unit L2, f3 is a focallength of the third lens unit L3, fw is a focal length of the entirezoom lens at the wide-angle end, and Bf is an air-conversion length ofback-focus (i.e., an air-conversion length from the lens surface closestto the reduction conjugate side to the reduction conjugate position).When light passes through material (lens material) having a refractiveindex greater than 1, the term air-conversion length refers to a valueobtained by dividing the length of the optical path in the material bythe refractive index of the material.

Condition (1) is for appropriately defining the refractive power of thethird lens unit L3. Condition (2) is for appropriately defining theratio of the refractive power of the third lens unit L3 to that of thesecond lens unit L2. Thus, it is possible to effectively achieve apredetermined zoom ratio while suppressing an increase in total lengthof the zoom lens. At the same time, it is possible to reduce aberrationvariation and provide a high level of optical performance throughout thezoom range.

In particular, as indicated by condition (2), the refractive power ofthe third lens unit L3 is made smaller than that of the second lens unitL2 and thus, aberration variation during zooming is reduced.

If the refractive power of the third lens unit L3 is greater than thatdefined by conditions (1) and (2), the amount of movement of the thirdlens unit L3 during zooming is reduced and the total length of the zoomlens is reduced. However, this causes an increase in aberrationvariation during zooming and makes it difficult to provide a high levelof optical performance throughout the zoom range. At the same time,since the refractive power of the second lens unit L2 is too small, theamount of movement of the second lens unit L2 during zooming isincreased. This is undesirable in that the total length of the zoom lensis increased.

If the upper limit of condition (3) is exceeded, the back focus of thezoom lens is too short. This causes less space for arranging, on thereduction conjugate side of the zoom lens, a mirror tilted to theoptical axis, a prism, and the like.

As described above, in the first, second, and third exemplaryembodiments, it is possible to provide a zoom lens having a long backfocus, offering a high degree of telecentricity and excellentimage-forming performance, and suitable for a projector.

In the first, second, and third exemplary embodiments, it is morepreferable that the following conditions be satisfied. Satisfying thefollowing conditions is not necessarily required for the presentinvention, but can add substantial value to the present invention.

If the zoom lens includes five lens units, it is preferable that thefollowing condition be satisfied:

|f4/fw|>15  (4)

where f4 is a focal length of a fourth lens unit L4.

The first lens unit L1 includes an aspherical lens element having anaspherical surface. It is preferable that at least one of the followingconditions be satisfied:

|f1/fw|>1.0  (5)

fASP/f1<3.0  (6)

where f1 is a focal length of the first lens unit L1 and fASP is a focallength of the aspherical lens element.

Condition (4) is for the zoom lens including five lens units. If therefractive power of the fourth lens unit L4 is too strong to satisfycondition (4), aberration variation during zooming increases and thus,it is difficult to provide a high level of optical performancethroughout the zoom range.

Condition (5) is for appropriately defining the negative refractivepower of the first lens unit L1, ensuring a predetermined back focusdistance, reducing the occurrence of negative distortion, andeffectively achieving a wider field angle. It is not preferable thatcondition (5) be not satisfied, because the degree of negativedistortion increases and the projected image will be distorted at thewide-angle end.

Condition (6) is for appropriately defining the negative refractivepower of the aspherical lens element in the first lens unit L1,effectively correcting various aberrations, and reducing the occurrenceof negative distortion at the wide-angle end, as in the case ofcondition (5). It is not preferable that condition (6) be not satisfied,because the degree of negative distortion increases and the projectedimage will be distorted at the wide-angle end.

In the first, second, and third exemplary embodiments, it is morepreferable that the numerical ranges of conditions (1) to (6) be definedas follows:

f3/fw>6.0  (1a)

f3/f2>3.0  (2a)

fw/Bf<0.6  (3a)

|f4/fw|>50  (4a)

|f1/fw|>1.5  (5a)

fASP/f1<2.5  (6a)

The above conditions (1a) to (6a) may be modified as follows:

f3/fw>11.0  (1b)

f3/f2>4.0  (2b)

fw/Bf<0.52  (3b)

|f4/fw|>120  (4b)

|f1/fw|>1.9  (5b)

fASP/f1<2.2  (6b)

It is still more preferable that the following conditions be satisfied:

50.0 (more preferably 20.0)>f3/fw  (1c)

12.0 (more preferably 6.0)>f3/f2  (2c)

0.28 (more preferably 0.40)<fw/Bf  (3c)

8000 (more preferably 6000)>|f4/fw|  (4c)

4.0 (more preferably 2.5)>|f1/fw|  (5c)

1.1 (more preferably 1.7)<fASP/f1  (6c)

Next, the zoom lens of each exemplary embodiment will be described indetail.

In the first exemplary embodiment illustrated in FIG. 1, there areprovided the first lens unit L1 of negative refractive power, the secondlens unit L2 of positive refractive power, the third lens unit L3 ofpositive refractive power, the fourth lens unit L4 of positiverefractive power, and the fifth lens unit L5 of positive refractivepower.

The first lens unit L1 includes a first lens sub-unit L1A of negativerefractive power and a first lens sub-unit L1B of positive refractivepower. The first lens sub-unit L1B serves as a focus lens. The fourthlens unit L4 corresponds to the middle lens unit LM. The fifth lens unitL5 corresponds to the last lens unit LR.

In the first exemplary embodiment, during zooming from the wide-angleend to the telephoto end, the second lens unit L2, the third lens unitL3, and the fourth lens unit L4 are independently moved toward thescreen surface S on the enlargement conjugate side (front side) asindicated by arrows. The second lens unit L2 and the third lens unit L3perform zooming, while the fourth lens unit L4 corrects image planevariation during zooming.

In the first exemplary embodiment, the second lens unit L2 performs aprimary zoom function. The same applies to the second and thirdexemplary embodiments described below.

The first lens unit L1 and the fifth lens unit L5 do not move forzooming. In the first lens unit L1, the first lens sub-unit L1B havingpositive refractive power and disposed closest to the reductionconjugate side is moved along the optical axis for focusing. The firstlens sub-unit L1A does not move during focusing.

With the zooming and focusing methods described above, the total lengthof the zoom lens (i.e., the distance from the first lens surface to thelast lens surface) can be kept constant (unchanged) even during zoomingand focusing. Thus, the robustness of the lens barrel structure can beimproved.

An aperture stop SP is located between the third lens unit L3 and thefourth lens unit L4. During zooming, the aperture stop SP moves togetherwith the third lens unit L3. Each lens surface is covered with amultilayer antireflective coating.

In the first exemplary embodiment, the first lens sub-unit L1A includes,in order from the enlargement conjugate side (front) to the reductionconjugate side (rear), a negative lens element G11 having a meniscusshape convex toward the front, a negative lens element G12 having ameniscus shape and being aspherical on both sides, a negative lenselement G13, and a positive lens element G14. The negative lens elementG13 and the positive lens element G14 are cemented together. The firstlens sub-unit L1B is constituted by a positive lens element G15 having ameniscus shape convex toward the rear.

The negative lens element G12 has aspherical surfaces on both sides tosuppress the occurrence of negative distortion.

In the first exemplary embodiment, the first lens sub-unit L1B on thereduction conjugate side (rear side) of the first lens unit L1 performsfocusing. Therefore, a curvature of field typical of a retrofocusconfiguration can be prevented from varying depending on the projectiondistance (from the lens surface closest to the enlargement conjugateside to the screen S). Thus, a high level of optical performance can bemaintained throughout the entire projection distance.

The second lens unit L2 is constituted by a biconvex positive lenselement G21.

The third lens unit L3 includes a cemented lens component formed bycementing together the positive lens element G31 having a biconvex shapeand the negative lens element G32, and the positive lens element G33having a meniscus shape convex toward the rear. With this configurationof the third lens unit L3, it is possible to reduce variation inlongitudinal chromatic aberration during zooming, reduce impact onoff-axis aberrations, and effectively correct spherical aberration.Thus, a high level of optical performance can be achieved throughout thezoom range.

The fourth lens unit L4 includes a cemented lens component formed bycementing together a biconcave negative lens element G41 and a biconvexpositive lens element G42, another cemented lens component formed bycementing together a biconcave negative lens element G43 and a positivelens element G44, and a biconvex positive lens element G45.

The fifth lens unit L5 is constituted by a biconvex positive lenselement G51. The fifth lens unit L5, which is the last lens unit LR, haspositive refractive power. Thus, telecentricity on the reductionconjugate side can be improved.

In the second exemplary embodiment illustrated in FIG. 3, there areprovided the first lens unit L1 of negative refractive power, the secondlens unit L2 of positive refractive power, the third lens unit L3 ofpositive refractive power, the fourth lens unit L4 of negativerefractive power, and the fifth lens unit L5 of positive refractivepower.

The second exemplary embodiment has the same configuration as that ofthe first exemplary embodiment except that the fourth lens unit L4 hasnegative refractive power.

In the third exemplary embodiment illustrated in FIG. 5, there areprovided the first lens unit L1 of negative refractive power, the secondlens unit L2 of positive refractive power, the third lens unit L3 ofpositive refractive power, the fourth lens unit L4 of negativerefractive power, the fifth lens unit L5 of positive refractive power,and a sixth lens unit L6 of positive refractive power.

The first lens unit L1 includes the first lens sub-unit L1A of negativerefractive power and the first lens sub-unit L1B of positive refractivepower. The first lens sub-unit L1B serves as a focus lens. The fourthlens unit L4 and the fifth lens unit L5 correspond to the middle lensunit LM. The sixth lens unit L6 corresponds to the last lens unit LR.

In the third exemplary embodiment, during zooming from the wide-angleend to the telephoto end, the second lens unit L2, the third lens unitL3, the fourth lens unit L4, and the fifth lens unit L5 areindependently moved toward the screen surface S on the enlargementconjugate side as indicated by arrows.

Thus, by moving the four lens units during zooming, a predetermined zoomratio can be easily obtained and aberration variation during zooming canbe reduced.

The first lens unit L1 and the sixth lens unit L6 do not move forzooming. In the first lens unit L1, the first lens sub-unit L1B havingpositive refractive power and disposed closest to the reductionconjugate side is moved along the optical axis for focusing. The firstlens sub-unit L1A does not move for focusing.

The aperture stop SP is located between the third lens unit L3 and thefourth lens unit L4. During zooming, the aperture stop SP moves togetherwith the third lens unit L3. Each lens surface is covered with amultilayer antireflective coating.

In the third exemplary embodiment, the lens configuration of the first,second, and third lens units L1, L2, and L3 is the same as that in thecase of the first exemplary embodiment. The fourth lens unit L4 of thethird exemplary embodiment is constituted by a cemented lens componentformed by cementing together the biconcave negative lens element G41 andthe biconvex positive lens element G42. The fifth lens unit L5 includesa cemented lens component formed by cementing together a biconcavenegative lens element G51 and a biconvex positive lens element G52, anda biconvex positive lens element G53. The sixth lens unit L6 isconstituted by a biconvex positive lens element G61.

In the third exemplary embodiment, where the zoom lens PL includes thefirst to sixth lens units L1 to L6, f4 is a combined focal length of themiddle lens unit LM (the fourth and fifth lens units L4 and L5) at thewide-angle end. In the first and second exemplary embodiments, where thezoom lens PL includes five lens units, the middle lens unit LMcorresponds to the fourth lens unit L4. In the third exemplaryembodiment, where the zoom lens PL includes six lens units, the middlelens unit LM corresponds to the fourth and fifth lens units L4 and L5.If the zoom lens PL includes seven or more lens units, all lens unitsdisposed on the rear side (reduction conjugate side) of the third lensunit L3 and on the front side (enlargement conjugate side) of the lastlens unit LR (closest to the reduction conjugate side) correspond to themiddle lens unit LM. That is, all lens units disposed on the rear side(reduction conjugate side) of the third lens unit L3 (i.e., the thirdlens unit from the enlargement conjugate side) and on the front side(enlargement conjugate side) of the last lens unit LR are included inthe middle lens unit LM.

With the configuration of any of the exemplary embodiments describedabove, it is possible to provide a zoom lens offering a high level ofoptical performance throughout the zoom range and suitable for aprojector.

FIG. 7 schematically illustrates a main part of an image projectionapparatus according to an exemplary embodiment of the present invention.

FIG. 7 illustrates an example in which the zoom lens of any one of theabove-described exemplary embodiments is used as a projection lens 103for a liquid crystal projector (image projection apparatus). In thisliquid crystal projector, image information corresponding to a pluralityof color light beams based on a plurality of (three) liquid crystaldisplay devices is synthesized by a color synthesizing unit and enlargedand projected by the projection lens 103 onto a screen (predeterminedsurface) 104.

In a color liquid crystal projector 101 illustrated in FIG. 7, red,green, and blue (RGB) color light beams from corresponding three liquidcrystal panels (liquid crystal display devices) 105R, 105G, and 105B aresynthesized into one optical path by a prism 102 serving as a colorsynthesizing unit. The liquid crystal panels 105R, 105G, and 105B mayeither be of transmissive or reflective type. The prism 102 may includea plurality of optical elements (e.g., dichroic mirror, dichroic prism,and polarizing beam splitter) and may further include a polarizer and awave plate between adjacent optical elements.

If condition (3) is satisfied in the color liquid crystal projector 101,a large image can be projected onto the screen 104 with a relativelyshort projection distance (from the lens surface closest to theenlargement conjugate side to the screen 104) and a long back focus canbe ensured. Therefore, a prism, a polarizer, and the like can be spacedout sufficiently for cooling. Conversely, if the upper limit ofcondition (3) is exceeded, the projection distance may be too long ordistances between adjacent components (e.g., prism block and polarizer)disposed on the reduction conjugate side may be too short for cooling.

Next, there will be shown numerical examples 1 to 3 corresponding to thefirst to third exemplary embodiments, respectively. In numericalexamples 1 to 3, i indicates the order of the optical surface from theenlargement side (front side), ri indicates the radius of curvature ofthe i-th optical surface (i-th surface), di indicates the distancebetween the i-th surface and the (i+1)-th surface, ni and vi indicatethe refractive index and Abbe number, respectively, of the material ofthe i-th optical member with respect to the d-line, and f indicates afocal length. A number marked with z indicates a distance that variesduring zooming. An optical surface marked with * is an asphericalsurface. Additionally, for example, “e−Z” means “10^(−Z)”.

In numerical examples 1 to 3, the four rearmost surfaces are surfaces ofthe glass block GB.

An aspherical shape is expressed as follows:

x=(h ² /R)/[1+[1−(1+k)(h/R)²]^(1/2) ]+Ah ⁴ +Bh ⁶ +Ch ⁸ +Dh ¹⁰ +Eh ¹²

where, with respect to a surface vertex, x is the amount of displacementin the direction of the optical axis at a height of h from the opticalaxis; R is a paraxial radius of curvature; k is a conical constant; andA, B, C, D, and E are aspherical coefficients.

The relationship of conditions 1 to 6 and numerical examples 1 to 3 willbe shown in Table 1.

f=21.8˜31.9(zoom ratio 1.47) ω=29.2°˜20.9°F/1.85˜F/2.65  [NumericalExample 1]

r d nd νd OBJ ∞ 2100.00  1 40.139 2.00 1.805 25.4  2 20.639 6.23  3*70.343 3.20 1.529 55.8  4* 27.417 12.41  5 −24.445 1.70 1.487 70.2  6320.652 5.45 1.800 34.9  7 −46.628 3.40  8 −47.281 2.85 1.834 37.1  9−40.415 31.87z 10 52.358 5.20 1.834 37.1 11 −669.376 25.31z 12 169.4544.80 1.487 70.2 13 −27.165 1.20 1.806 33.2 14 −173.752 1.94 15 −44.5592.65 1.805 25.4 16 −32.271 0.50 17 aperture stop 1.92z 18 −81.130 1.201.800 34.9 19 22.436 7.30 1.589 61.1 20 −34.006 1.80 21 −22.432 1.301.800 34.9 22 66.267 6.10 1.487 70.2 23 −38.768 1.22 24 143.991 8.901.496 81.5 25 −30.799 0.70z 26 84.252 3.85 1.805 25.4 27 −290.436 1.3028 ∞ 34.60 1.516 64.1 29 ∞ 4.00 30 ∞ 21.00 1.805 25.4 31 ∞ W M T d931.87 20.11 7.35 d11 25.31 23.40 14.29 d17 1.92 7.61 22.16 d25 0.70 8.6816.00 z4 z5 z6 z7 d0 1200.00 8700.00 1200.00 8700.00 d7 4.27 2.50 4.272.50 d9 31.01 32.78 6.48 8.25 K A B C D E 3 0.000e+000 2.994e−005−8.530e−008 2.529e−010 −3.394e−013 1.099e−016 4 0.000e+000 2.050e−005−9.984e−008 1.809e−010 −1.919e−014 −8.454e−016

f=21.8˜31.9(zoom ratio 1.47) ω=29.2°˜20.9°F/1.85˜F/2.65  [NumericalExample 2]

r d nd νd OBJ ∞ 2100.00  1 39.964 2.00 1.805 25.4  2 20.625 6.39  3*70.343 3.20 1.529 55.8  4* 27.417 12.33  5 −24.762 1.70 1.487 70.2  6268.348 5.58 1.800 34.9  7 −48.704 3.47  8 −49.535 3.09 1.834 37.1  9−41.449 31.81z 10 51.526 5.09 1.834 37.1 11 −1038.197 25.25z 12 101.1644.92 1.487 70.2 13 −29.190 1.20 1.806 33.2 14 −591.118 2.05 15 −49.4092.80 1.805 25.4 16 −33.951 0.50 17 aperture stop 1.89z 18 −81.200 1.201.800 34.9 19 22.137 7.21 1.589 61.1 20 −35.930 1.93 21 −22.287 1.301.800 34.9 22 69.202 6.18 1.487 70.2 23 −37.236 0.61 24 137.580 8.771.496 81.5 25 −30.718 0.70z 26 88.953 3.85 1.805 25.4 27 −229.280 1.3028 ∞ 34.60 1.516 64.1 29 ∞ 4.00 30 ∞ 21.00 1.805 25.4 31 ∞ W M T d931.81 20.09 7.36 d11 25.25 23.25 14.24 d17 1.89 7.74 22.18 d25 0.70 8.5815.86 z4 z5 z6 z7 d0 1200.00 8700.00 1200.00 8700.00 d7 4.26 2.64 4.262.64 d9 31.02 32.64 6.57 8.19 K A B C D E 3 0.000e+000 2.994e−005−8.530e−008 2.529e−010 −3.394e−013 1.099e−016 4 0.000e+000 2.050e−005−9.984e−008 1.809e−010 −1.919e−014 −8.454e−016

f=21.8˜31.9(zoom ratio 1.47) ω=29.2°˜20.9°F/1.85˜F/2.65  [NumericalExample 3]

r d nd νd OBJ ∞ 2100.00  1 40.155 2.00 1.805 25.4  2 20.604 6.48  3*70.343 3.20 1.529 55.8  4* 27.417 12.04  5 −24.965 1.70 1.487 70.2  6201.643 5.61 1.800 34.9  7 −49.877 3.40  8 −51.434 3.04 1.834 37.1  9−42.602 31.80z 10 52.101 5.09 1.834 37.1 11 −817.882 25.45z 12 99.4074.87 1.487 70.2 13 −29.364 1.20 1.806 33.2 14 −645.940 2.02 15 −50.1742.70 1.805 25.4 16 −34.227 0.50 17 aperture stop 1.86z 18 −74.947 1.201.800 34.9 19 22.829 7.18 1.589 61.1 20 −35.999 2.00z 21 −22.268 1.301.800 34.9 22 72.709 6.44 1.487 70.2 23 −35.549 0.64 24 151.375 8.751.496 81.5 25 −30.807 0.70z 26 82.937 3.85 1.805 25.4 27 −299.797 1.3028 ∞ 34.60 1.516 64.1 29 ∞ 4.00 30 ∞ 21.00 1.805 25.4 31 ∞ W M T d931.80 19.21 7.38 d11 25.45 22.88 14.42 d17 1.86 8.26 21.71 d20 2.00 2.112.49 d25 0.70 9.34 15.80 z4 z5 z6 z7 d0 1200.00 8700.00 1200.00 8700.00d7 4.19 2.57 4.19 2.57 d9 31.01 32.63 6.58 8.20 K A B C D E 3 0.000e+0002.994e−005 −8.530e−008 2.529e−010 −3.394e−013 1.099e−016 4 0.000e+0002.050e−005 −9.984e−008 1.809e−010 −1.919e−014 −8.454e−016

Numerical Example Condition 1 2 3 (1) f3/fw 12.80 12.80 12.71 (2) f3/f24.80 4.78 4.73 (3) fw/Bf 0.490 0.488 0.488 (4) |f4/fw| 3671 148.3 5188(5) |f1/fw| 2.07 2.08 2.08 (6) fASP/f1 1.92 1.91 2.04

According to the exemplary embodiments described above, it is possibleto provide a zoom lens capable of effectively correcting variousaberrations during zooming, offering a high level of optical performancethroughout the zoom range, and suitable for a projector (imageprojection apparatus). Additionally, it is possible to provide an imageprojection apparatus including the zoom lens and capable of projecting,onto a projection surface (e.g., screen), a high-definition image(modulated image light) formed on an image display device (e.g., liquidcrystal panel).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications and equivalent structures and functions.

1. A zoom lens comprising, in order from an enlargement conjugate sideto a reduction conjugate side: a first lens unit of negative refractivepower; a second lens unit of positive refractive power; a third lensunit of positive refractive power; a middle lens unit including at leastone lens unit; and a last lens unit of positive refractive power,wherein all the lens units, except the first lens unit and the last lensunit, move during zooming, and wherein the following conditions aresatisfied:f3/fw>4.0f3/f2>2.0fw/Bf<0.6 where f2 is a focal length of the second lens unit, f3 is afocal length of the third lens unit, fw is a focal length of the entirezoom lens at a wide-angle end, and Bf is an air-conversion length ofback-focus.
 2. The zoom lens according to claim 1, wherein the middlelens unit includes a fourth lens unit of positive or negative refractivepower, and wherein the following condition is satisfied:|f4/fw|>15 where f4 is a focal length of the fourth lens unit.
 3. Thezoom lens according to claim 1, wherein the middle lens unit includes,in order from the enlargement conjugate side to the reduction conjugateside, a fourth lens unit of negative refractive power and a fifth lensunit of positive refractive power, and wherein the following conditionis satisfied:|f4/fw|>15 where f4 is a focal length of the fourth lens unit.
 4. Thezoom lens according to claim 1, wherein the following condition issatisfied:|f1/fw|>1.0 where f1 is a focal length of the first lens unit.
 5. Thezoom lens according to claim 1, wherein the first lens unit has anaspherical lens element with an aspherical lens surface, and wherein thefollowing condition is satisfied:fASP/f1<3.0 where f1 is a focal length of the first lens unit and fASPis a focal length of the aspherical lens element.
 6. The zoom lensaccording to claim 1, wherein the third lens unit includes a cementedlens component formed by cementing together a positive lens element anda negative lens element, and a meniscus positive lens element convextoward the reduction conjugate side.
 7. The zoom lens according to claim1, wherein in the first lens unit, a lens element closest to thereduction conjugate side is a focus lens that moves in an optical axisdirection for focusing.
 8. The zoom lens according to claim 1, whereinthe first lens unit consists of, in order from the enlargement conjugateside to the reduction conjugate side, two meniscus negative lenselements, a cemented lens component formed by cementing together anegative lens element and a positive lens element, and a meniscuspositive lens element; the second lens unit consists of a biconvexpositive lens element; and the last lens unit consists of a biconvexpositive lens element.
 9. The zoom lens according to claim 1, wherein adisplay image located on the reduction conjugate side is projected ontoa predetermined surface.
 10. An image projection apparatus comprising: adisplay device configured to form an original image; and a zoom lensconfigured to project the original image onto a projection surface andincluding, in order from the projection surface side toward the displaydevice, a first lens unit of negative refractive power; a second lensunit of positive refractive power; a third lens unit of positiverefractive power; a middle lens unit including at least one lens unit;and a last lens unit of positive refractive power, wherein all the lensunits, except the first lens unit and the last lens unit, move duringzooming, and wherein the following conditions are satisfied:f3/fw>4.0f3/f2>2.0fw/Bf<0.6 where f2 is a focal length of the second lens unit, f3 is afocal length of the third lens unit, fw is a focal length of the entirezoom lens at a wide-angle end, and Bf is an air-conversion length ofback-focus.
 11. A zoom lens comprising, in order from an enlargementconjugate side to a reduction conjugate side: a first lens unit ofnegative refractive power; a second lens unit of positive refractivepower; a third lens unit of positive refractive power, the third lensunit includes a positive lens element, a negative lens element, and ameniscus positive lens element convex toward the reduction conjugateside in order from an enlargement conjugate side to a reductionconjugate side; a middle lens unit including at least one lens unit; anda last lens unit of positive refractive power, wherein all the lensunits, except the first lens unit and the last lens unit, move duringzooming, and wherein the following conditions are satisfied:f3/fw>4.0f3/f2>2.0fw/Bf<0.8 where f2 is a focal length of the second lens unit, f3 is afocal length of the third lens unit, fw is a focal length of the entirezoom lens at a wide-angle end, and Bf is an air-conversion length ofback-focus.
 12. An image projection apparatus comprising: a displaydevice configured to form an original image; and a zoom lens configuredto project the original image onto a projection surface and including,in order from the projection surface side toward the display device, afirst lens unit of negative refractive power; a second lens unit ofpositive refractive power; a third lens unit of positive refractivepower, the third lens unit includes a positive lens element, a negativelens element, and a meniscus positive lens element convex toward thereduction conjugate side in order from an enlargement conjugate side toa reduction conjugate side; a middle lens unit including at least onelens unit; and a last lens unit of positive refractive power, whereinall the lens units, except the first lens unit and the last lens unit,move during zooming, and wherein the following conditions are satisfied:f3/fw>4.0f3/f2>2.0fw/Bf<0.8 where f2 is a focal length of the second lens unit, f3 is afocal length of the third lens unit, fw is a focal length of the entirezoom lens at a wide-angle end, and Bf is an air-conversion length ofback-focus.